Interferential dielectric mirrors comprise a substrate covered by a dielectric film which reflects one or more desired wavelengths, while having a relatively low intrinsic absorption compared with the conventionally used metals.
Preferably, the invention relates to monochroic or polychroic, passive, dielectric mirrors reflecting wavelengths from the near ultraviolet to the near infrared.
Interferential dielectric mirrors can more specifically be used in high power laser systems, in photovoltaic and thermal applications, solar applications or in integrated optical systems.
A certain number of processes are already known making it possible to produce transparent, dielectric films and to deposit them on non-metallic substrates.
German patents 736 411 and 937 913 refer to the use of hydrolyzable compounds for the preparation of various interference films. The major disadvantage of these processes is the indispensable heat treatment at between 500.degree. and 600.degree. C. in order to convert the polymeric intermediates into the final, dense ceramics. These high temperatures limit the choice of the nature of the substrate to be coated.
U.S. Pat. No. 2,584,905 deals with the preparation of thin, reflecting films from alcoholic TiCl.sub.4 solutions and a silicon alkoxide. Once again, it is necessary to have a heat treatment stage making it possible to appropriately densify the oxides. In this process, the problems of glazing and scaling linked with the densification of the materials, considerably reduce the production of high reflection, multilayer structures.
U.S. Pat. No. 3,460,956 describes the preparation of reflecting TiO.sub.2 films from hydrolyzates of tetraalkyl titanates in an alcoholic medium. Once again, for effectively converting the polymeric film into a dense oxide, said film must undergo heating at around 500.degree. C., which is therefore prejudicial for any organic substrate.
U.S. Pat. Nos. 2,768,909 and 2,710,267 describe the production of reflecting TiO.sub.2 films from alcoholic sols of a titanium alkoxide, said sols being hydrolyzable by atmospheric moisture. This approach also requires a high baking of the condensed intermediates.
U.S. Pat. No. 4,272,588 relates to the possibility of increasing the reflectivity of precious metal mirrors, as well as the possibility of making the same chemically passive by the deposition of TiO.sub.2 and Ta.sub.2 O.sub.5 dielectric coatings obtained from molecular precursors. Such layers are obtained by an obligatory heating at approximately 400.degree. C.
The use of submicroscopic, colloidal particles for the preparation of microporous, optical films dates back about forty years and was in particular described in U.S. Pat. No. 2,432,484. The latter document describes the use of SiO.sub.2, colloids synthesized from a silicon alkoxide in solution, and/or a sodium silicate, in order to obtain antireflection films. Roughly 35 years later, another U.S. Pat. No. 4,271,210 describes the use of the colloidal procedure for producing thin optical films. The process described in said patent consists of synthesizing and depositing microgranular films of hydrated alumina as an antireflection coating on a vitreous substrate. This process involves heat treatment at between 300.degree. and 500.degree. C. in order to obtain the metal oxide, which limits the substrates which can be used.
Moreover, processes for the deposition of colloidal layers are known. Thus, an article entitled "Colloidal Sol-Gel Optical Coatings" published in "The American Ceramic Society Bulletin", vol. 69, no. 7, pp. 1141 to 1143, 1990, describes the possibility of depositing several layers of colloidal materials by centrifugal coating, in order to produce the optical components of a laser. This article states that by using sol-gel colloidal suspensions and by appropriately choosing volatile solvents for forming the liquid phase of the colloidal medium, it is possible to carry out treatments at ambient temperature without excessive heating of the substrate. Therefore this procedure makes it possible to treat thermally fragile materials.
Moreover, in the field of replicating optical surfaces, the preparation of metal mirrors goes back more than 40 years. U.S. Pat. No. 2,444,533 relates to a surface copying process by molding--demolding in order to obtain concave substrates of optical quality.
French patent 2 061 847 relates to the chemical replication of optical surfaces for the production of planar metal mirrors.
French patents 2 310 308 and 2 310 477 describe the production of replicated metal reflectors for electricity generating devices by concentrating solar energy.
Finally, a surface replication method with epoxy resins and using a metallic mould release or demoulding agent is described in the article by Assus (J. of Optics, vol. 20, no. 5. pp. 219-223, 1989), while a moulding process with a resin polymerizing under the action of ultraviolet rays and a chemical demolding agent is described in the article by Mahe and Marioge (J. of Optics, vol. 19, no. 2, pp. 83-91, 1988).
Most of the methods relating to the reflecting treatment of surfaces referred to hereinbefore suffer from the disadvantage of requiring heating at high temperatures and therefore limiting the substrates to which this treatment type can be applied. Moreover, none of the prior art methods makes it possible by a process in solution, to produce dielectric mirrors simultaneously having a good resistance to the laser flux, a large surface and low manufacturing costs. Moreover, no process is known which makes it possible to produce dielectric mirrors replicated in large numbers, with relatively unsophisticated equipment and with low production costs.